Key square

In the normal cases of the king and pawn versus king ending, a key square is a square that lets the pawn's king force promotion with accurate play once the king occupies it, regardless of whose turn it is at that moment. The square is not permanently special; its role is relative to the pawn's location and progress.

The concept replaces a long move sequence with a concrete target. Instead of asking only whether the pawn can advance, the attacker asks whether the king can reach one of the squares that guarantees future progress. Opposition is often the method used to reach a key square or prevent the opposing king from doing so.

Locating key squares for a non-edge pawn

• When the pawn is on its second, third, or fourth horizontal row, counted from the side where it started, the three key squares are two rows ahead: the square directly ahead and the two neighboring squares beside it.
• When the pawn is on its fifth or sixth row, there are six key squares: the three squares on the next row and the three on the row beyond it.
• When the pawn is on its seventh row, the adjoining squares on the seventh and eighth rows are key squares.

A rook pawn, which starts on an edge column, is an important exception. The attacking king has less room to go around the defender, and a defending king established in the promotion corner may secure a draw. The ordinary pattern of three or six key squares should therefore not be applied mechanically to rook pawns.

There is also a narrow exception for a knight pawn, the pawn that begins one column in from the edge. If it has reached its sixth row, the defending king is in the corner beside the promotion square, and the defender is to move, some squares in the usual pattern may produce stalemate rather than a win. Stalemate is a draw because the player to move has no legal move while the king is not in check. The exact move order must be checked.

Some books use key square and critical square as synonyms. In this glossary, key square has the precise king-and-pawn versus king meaning, while critical square may refer more broadly to a decisive square in another ending.